Industrial coating compositions are used in a wide variety of applications, including application directly to metal, to primed metals and a number of other materials including plastics etc. Both liquid and powder coatings are common and are applied by a variety of methods including, for liquids, electrodeposition, spraying, extruding, plate coating, dipping, and coil coating etc., and for powders, cloud chamber, plasma coating, electrostatic deposition and the like. Coating films having a thickness of from 0.01 and 5 mils are typically employed.
The conditions under which coatings are applied, and the conditions under which they are cured vary widely. For example, the coating can be applied under ambient conditions but higher temperatures are often used and temperatures up to about 450° F. (232° C.), or higher are employed during application. Cure temperatures also vary widely, and depending on the coating composition temperatures of up to 400° C. and higher, e.g., from about 60° C. to 400° C., may be encountered. The length of time required for cure can also vary, e.g., from about 10 sec. to about 60 minutes is typical.
Coatings that cure more efficiently, i.e., quickly, and which use less energy, i.e., cure at lower temperatures, are generally in demand.
The present invention is directed to a coating composition that cures at lower temperature and with shorter cure times than other currently used coatings due to the presence of select metal sulfonate catalysts, and to a method for decreasing the bake temperature and cure or dwell time of a coating process by utilizing a coating composition comprising metal sulfonate catalysts. The catalysts disclosed in this invention are effective in a number of industrial coating applications including automotive coatings and coatings over temperature sensitive substrates such as plastics.
Coil coatings are an example of one important industrial coating application. Coil coatings are applied to coiled sheet metal stock, such as steel or aluminum, in an economical, high speed process. The coil coating process results in a high quality, uniform coating with little waste of the coating and little generation of organic emissions as compared to other coating methods, e.g. spray application of a coating composition.
Coil coating is a continuous feeding operation, with the end of one coil typically being joined (e.g., stapled) to the beginning of a next coil. The coil is first fed into an accumulator tower and after coating is fed into an exit accumulator tower, with the accumulator towers allowing the coating operation to continue at constant speed even when intake of the steel is delayed, for example, to start a new roll, or winding of the steel after coating is delayed, for example, to cut the steel to end one roll and begin a new roll. The coil is generally cleaned to remove oil or debris, pre-treated, primed with a primer on both sides, baked to cure the primer, quenched to cool the metal, and then coated on at least one side with a topcoat. A separate backer or a different topcoat may be applied on the other side. The topcoat is baked and quenched, then fed into the exit accumulator tower and from there is re-rolled.
One of the controlling factors for the coil coating line speed is the oven dwell time necessary to cure the applied coating at the cure oven temperature. A coating composition that can be cured in a shorter time at cure temperature allows a faster and more economical coil coating process. A number of other properties are also important for coil coatings, such as resistance to degradation on outdoor exposure (weatherability), chemical resistance, water resistance, scratch resistance, gloss, hardness, and resistance to delamination when the substrate is bent. The bending property is important because after being coated the metal is subjected to a forming step. For example, building panels are formed into a three-dimensional shape after coating. It is important that the coating not lose adhesion during the forming step or steps. Weatherability is important for metal that will be used for building panels, gutters, garage doors, sign stock, panels used for vehicle parts, or other such uses where the coated surface is exposed to outdoor weather and sun. While the bending property is generally better with softer, more flexible binders, weatherability and other durability properties are generally better with harder binders.
In the coil coating operation, a coil of sheet metal is uncoiled as it is pulled through a series of rollers, one or more of which is a paint applicator roller, at up to about 600 feet per minute. It is then passed through a curing oven and coiled again for the market. The paint is picked up by a roller rotating in the paint pan and transferred to a reverse or direct applicator roller. The cure temperature in a coil coating operation is typically measured as a peak metal temperature (PMT). The peak metal temperature is generally between 425° F. (218° C.) and 525° F. (274° C.).
Various coil coating compositions providing different coating properties are known. For example U.S. Pat. No. 6,413,648 discloses a thermosetting coating composition containing a mixture of two polymers selected from linear or branched polyacrylates or polyesters, one of which is amorphous with a glass transition temperature greater than about 45° C.
U.S. Pat. No. 5,563,223 discloses a coil coating composition that balances processability of the coating with the need for alkali resistance, gasket resistance, weatherability and resistance to staining after cure, the composition comprising a curing agent and a polyester prepared from an acid component that is at least 50 mole % aromatic dicarboxylic acid and a glycol component having 1-25 mole % 2-methyl-1,3-propanediol and 75-99 mole % alkylene glycol having 5 to 10 carbon atoms. Alternatively, the glycol component can be 20-85 mole % of alicyclic glycol, 80-15 mole % of the addition product of bisphenol A and alkylene oxide, and up to 50 mole % of other glycol(s).
U.S. Pat. No. 5,380,816 discloses thermoset coating compositions comprising linear polyesters consisting essentially of recurring units of isophthalic acid, an aliphatic diol component including 2-methyl-1,3-propanediol, and, optionally a further dicarboxylic acid. The cured coating reportedly has improved flexibility and hardness, although it requires a relatively long cure time for a coil coating.
U.S. Pat. No. 4,968,775 discloses a thermosetting coil coating composition resistant to crystallization comprising an aminoplast resin and a polyester prepared by condensation of 2-methyl-1,3-propanediol, neopentyl glycol, isophthalic acid, and terephthalic acid, and may contain 1,6-hexanediol or other symmetrical glycol, trimethylolpropane, adipic acid or other symmetrical aliphatic dicarboxylic acid, and/or trimellitic anhydride.
U.S. Pat. No. 4,734,467 discloses a coil coating composition consisting essentially of a crosslinking component selected from melamine resin or isocyanate compound and a mixture of linear and branched polyester resins. The cured coating is reported to have desirable hardness, bending, processability, fastness to boiling water, weather resistance, chemical resistance, and marker stain resistance.
U.S. Pat. No. 6,897,265 discloses a coil coating composition with excellent properties applied at a lower peak metal temperature comprising (a) a first, branched polyester prepared from a polyol selected from a flexibilizing diol, 2-methyl-1,3-propanediol, and a polyol having at least three hydroxyl groups and isophthalic acid; (b) a second, linear polyester prepared from the polyol component selected from a flexibilizing diol and 2-methyl-1,3-propanediol and isophthalic acid; and (c) a crosslinking agents including, aminoplasts and isocyanates.
Catalysts are often employed in the curing of coiled coatings for example, conventional acid catalysts such as aromatic sulfonic acid catalysts, including napthalene disulfonic acid, dinonyl napthalene sulfonic acid, para-toluene sulfonic acid, and dececylbenzene sulfonic acid, other acids such as phosphate acid catalysts including phosphoric acid, and mono- and dibutyl acid phosphate may also be used. Blocked acid catalysts, such as epoxy and amine blocked sulfonic acid catalysts are also known, but often suffer from drawbacks such as popping or undue color development.
The attempts of the art cited above focused on adjusting the coating in order to provide lower temperature cure in order to prevent discoloration. However, these coatings still suffered from other drawbacks of the prior art including popping, amine migration and discoloration. The changing of the polymer chemistry did not address the problems with the catalyst.
U.S. Pat. No. 8,431,730, incorporated herein by reference, discloses latent sulfonate ester catalysts that offer improvement over the epoxy and amine blocked catalysts in coatings such as those comprising calcium anti-corrosive pigments, however, improvements in catalysts, generally useful in a variety of coil coatings are still needed.
Another example of an important industrial coating includes multi-layer coatings prepared by sequentially applying different functional layers, e.g., a primer surfacer, a base coat composition, and/or a clear coat composition, on a substrate in a wet-on-wet manner, and simultaneously curing the layers together in a single baking step. The resulting multi-layered coating film has excellent aesthetic appearance, strike-in resistance, chipping resistance, sag resistance, and film build even when formed in a three wet layered application method.
Composite color-plus-clear coatings are widely utilized multi-layered coatings. They are particularly desirable where exceptional gloss, depth of color, distinctness of image, or special metallic effects are required. The automotive industry has made extensive use of color-plus-clear composite coatings for automotive body panels.
Typically, composite color-plus-clear coatings are coating systems requiring the application of a first coating, typically a colored basecoat coating, followed by the application of a second coating, generally a clearcoat, over the noncured or “wet” first coating. The applied first and second coatings are then cured. Thus, such systems are often described as “wet on wet” or “two coat/one bake”. Drying processes which fall short of complete cure may be used between the applications of the coatings. In many applications the base coat is applied to a primer surfacer typically comprising as a film forming binder a highly branched acrylic polymer having a hydroxyl, carboxyl and/or other crosslinkable functional group and an aminoplast resin crosslinking agent.
Clearcoats used in color-plus-clear systems must have an extremely high degree of clarity in order to achieve the desired visual effects. High gloss coatings also require a low degree of visual aberrations at the surface in order to achieve the desired visual effect such as high distinctness of image (DOI). As a result, clearcoats of color-plus-clear systems are especially susceptible to the phenomenon known as environmental etch, i.e., spots or marks on or in the clear finish that often cannot be rubbed out.
Surface imperfections and/or defects can occur during the multistep application process typically used to apply composite coatings. Such surface imperfections and/or defects are sometimes not repairable until after the curing of the composite coatings. In some instances, the repair process occurs subsequent to the addition of other components to a coated article. The additional components may have melting or deformation temperatures which are lower than the cure temperature of the original composite coating.
Ideally, it would be desirable to repair surface imperfections and/or defects with the original composite coating or components thereof, in order to obtain uniform appearance and performance properties over the whole of the coated article. In particular, it would desirable to have a repair coating which provides the same performance and appearance properties of the original composite coating or components thereof. However, the cure schedule for traditional composite coatings typically requires temperatures greater than the melt or deformation temperature of some article components added subsequent to the original composite coating application process.
Thus, there is a need for a curable coating composition suitable for use in low bake repair of color-plus-clear composite coatings or coating components thereof, in particular, one which can be used in low bake repair of color-plus clear composite coatings or components thereof, which provides desirable performance and appearance properties and cures at a temperature less than that of the cure temperature required for the original color-plus-clear composite coating.
It has been found that coatings containing metal sulfonate latent catalysts of the present invention can be beneficially used as coil coatings, as any or all of the layers in multi-layer coatings, curable coating compositions for use in low bake repair of color-plus clear composite coatings, etc., which coatings offer improvements in cure time, lower cure temperature and frequently better coating properties. The catalysts of the invention avoid the drawbacks of epoxy and amine blocked acid catalysts and provide benefits in a wide variety of coating applications.